The invention relates generally to reflow soldering and more specifically to a method for improving the reliability of solder joints for surface mount devices.
The size and shape of solder joints for surface mount components are typically used as indicators of the reliability of the solder joint. It is common in many engineering standards such as IPC-A-610A for a solder joint having a concave cross section to be the only acceptable solder joint shape. As an example, U.S. Pat. No. 5,062,567 describes a convex solder joint as unacceptable for a component having a lead designed to encourage a concave shape at the lead.
In surface mount design, the solder pedestal (i.e., the area of the solder joint between the bottom of the component terminal and the solder pad) is recognized as an area where relatively high stresses arise under operating conditions. Stresses in this area contribute to the failure of the joints, reducing product reliability. One method of reducing these stresses is to prevent solder from entering the area between the component and the pad. However, such designs are often difficult to manufacture consistently and are often costly.
Solder joints produced under industry-standard design and process conditions typically have a concave cross-section after reflow. Concave solder joints indicate that a net downward force acted on the component during reflow. This downward force causes the component to sink in the molten solder to a position close to the surface of the circuit board, resulting in a low pedestal height. One disadvantage of a low pedestal height is that stresses in the pedestal are larger for lower pedestal heights.
It is desirable to provide a reliable solder joint having a controlled shape which is relatively easy to manufacture, especially for larger components. In addition, it is desirable to increase the component density on a circuit board.